Up to the present, capacitors, particularly electric double layer capacitors having a capacity intermediate between that of a battery and that of a capacitor, as a backup power supply for IC's and memories and a supplement or alternative to secondary batteries, have been used widely as a low-power direct-current power supply. In recent years, however, there have been new portable electronic devices coming into being one after another, such as VTR with a built-in camera, cellular phones and laptop computers. Amid this trend, there has been a growing demand that those electric double layer capacitors used as a backup power supply and a supplement or alternative to secondary batteries should have higher energy density.
These electric double layer capacitors, unlike batteries, do not convert a chemical change into electric energy. They utilize a large capacity of the electric double layer that occurs between the electrodes and the electrolytic solution to put in and take out an electric charge in the same manner as the charge and discharge of batteries. Such electric double layer capacitors are made up of two electrodes, a separator and, normally a corrosion-resistant electrolytic solution. The two electrodes, formed from a material having a large surface area such as activated carbon and a binder such as a fluororesin, are so arranged that they face each other via a porous separator made of polyethylene or polypropylene, and the space between these electrodes and the porous separator and the inside of the porous separator are filled with the aforementioned electrolytic solution.
As the electrolytic solution for such electric double layer capacitors, aqueous-solution-based electrolytic solutions and organic-solvent-based electrolytic solutions (non-aqueous electrolytic solutions) are used. However, aqueous-solution-based electrolytic solutions have the problem of it being difficult to obtain a low potential window of electrochemical stability(approx. 1.2 V) and high-energy-density electric double layer capacitors from them.
By contrast, compared with aqueous-solution-based electrolytic solutions, organic-solvent-based electrolytic solutions (non-aqueous electrolytic solutions) show a high potential window of electrochemical stability and therefore make the formation of high-energy-density capacitors possible. Because of this, electric double layer capacitors using non-aqueous electrolytic solutions are beginning to come into use rapidly as backup power supplies for electronic equipment for non-military use.
As such non-aqueous electrolytic solutions, a mixture of a non-aqueous solvent such as a cyclic carbonic ester exhibiting a high dielectric constant and an electrolyte such as tetraethylammoniumtetrafluoroborate, for example, is used.
However, such an electrolytic solution as mentioned above has had the problem of releasing a large amount of gas due to the decarboxylation decomposition of the solvent at high temperature due to the use of cyclic carbonic ester as the solvent and consequently being unable to provide long-life capacitors. Furthermore, when future capacitors have high energy density, the aforementioned electrolytic solutions might be insufficient in the potential window of electrochemical stability. Because of this, it has been hoped that a non-aqueous electrolyte having better charging/discharging cycle properties will come into existence.